Autonomous Synthesis of Thin Film Materials with Pulsed Laser Deposition Enabled by In Situ Spectroscopy and Automation

Autonomous systems that combine synthesis, characterization, and artificial intelligence can greatly accelerate the discovery and optimization of materials, however platforms for growth of macroscale thin films by physical vapor deposition techniques have lagged far behind others. Here this study demonstrates autonomous synthesis by pulsed laser deposition (PLD), a highly versatile synthesis technique, in the growth of ultrathin WSe2 films. By combing the automation of PLD synthesis and in situ diagnostic feedback with a high-throughput methodology, this study demonstrates a workflow and platform which uses Gaussian process regression and Bayesian optimization to autonomously identify growth regimes for WSe2 films based on Raman spectral criteria by efficiently sampling 0.25% of the chosen 4D parameter space. With throughputs at least 10x faster than traditional PLD workflows, this platform and workflow enables the accelerated discovery and autonomous optimization of the vast number of materials that can be synthesized by PLD.

Anomalous isotope effect on the optical bandgap in a monolayer transition metal dichalcogenide semiconductor

Isotope effects have received increasing attention in materials science and engineering because altering isotopes directly affects phonons, which can affect both thermal properties and optoelectronic properties of conventional semiconductors. However, how isotopic mass affects the optoelectronic properties in 2D semiconductors remains unclear because of measurement uncertainties resulting from sample heterogeneities. Here, we report an anomalous optical bandgap energy red shift of 13 (±7) milli-electron volts as mass of Mo isotopes is increased in laterally structured 100MoS2-92MoS2 monolayers grown by a two-step chemical vapor deposition that mitigates the effects of heterogeneities. This trend, which is opposite to that observed in conventional semiconductors, is explained by many-body perturbation and time-dependent density functional theories that reveal unusually large exciton binding energy renormalizations exceeding the ground-state renormalization energy due to strong coupling between confined excitons and phonons. The isotope effect on the optical bandgap reported here provides perspective on the important role of exciton-phonon coupling in the physical properties of two-dimensional materials.

Tuning Ionic Conductivity in Fluorite Gd-Doped CeO2-Bixbyite RE2O3 (RE = Y and Sm) Multilayer Thin Films by Controlling Interfacial Strain

Interfacial strain in heteroepitaxial oxide thin films is a powerful tool for discovering properties and recognizing the potential of materials performance. Particularly, facilitating ion conduction by interfacial strain in oxide multilayer thin films has always been seen to be a highly promising route to this goal. However, the effect of interfacial strain on ion transport properties is still controversial due to the difficulty in deconvoluting the strain contribution from other interfacial phenomena, such as space charge effects. Here, we show that interfacial strain can effectively tune the ionic conductivity by successfully growing multilayer thin films composed of an ionic conductor Gd-doped CeO2 (GDC) and an insulator RE2O3 (RE = Y and Sm). In contrast to compressively strained GDC-Y2O3 multilayer films, tensile strained GDC-Sm2O3 multilayer films demonstrate the enhanced ionic conductivity of GDC, which is attributed to the increased concentration of oxygen vacancies. In addition, we demonstrate that increasing the number of interfaces has no impact on the further enhancement of the ionic conductivity in GDC-Sm2O3 multilayer films. Our findings demonstrate the unambiguous role of interfacial strain on ion conduction of oxides and provide insights into the rational design of fast ion conductors through interface engineering.

Real-Time Diagnostics of 2D Crystal Transformations by Pulsed Laser Deposition: Controlled Synthesis of Janus WSSe Monolayers and Alloys

Energetic processing methods such as hyperthermal implantation hold special promise to achieve the precision synthesis of metastable two-dimensional (2D) materials such as Janus monolayers; however, they require precise control. Here, we report a feedback approach to reveal and control the transformation pathways in materials synthesis by pulsed laser deposition (PLD) and apply it to investigate the transformation kinetics of monolayer WS₂ crystals into Janus WSSe and WSe₂ by implantation of Se clusters with different maximum kinetic energies (<42 eV/Se-atom) generated by laser ablation of a Se target. Real-time Raman spectroscopy and photoluminescence are used to assess the structure, composition, and optoelectronic quality of the monolayer crystal as it is implanted with well-controlled fluxes of selenium for different kinetic energies that are regulated with in situ ICCD imaging, ion probe, and spectroscopy diagnostics. First-principles calculations, XPS, and atomic-resolution HAADF STEM imaging are used to understand the intermediate alloy compositions and their vibrational modes to identify transformation pathways. The real-time kinetics measurements reveal highly selective top-layer conversion as WS₂ transforms through WS_2(1-x)Se_2x alloys to WSe₂ and provide the means to adjust processing conditions to achieve fractional and complete Janus WSSe monolayers as metastable transition states. The general approach demonstrates a real-time feedback method to achieve Janus layers or other metastable alloys of the desired composition, and a general means to adjust the structure and quality of materials grown by PLD, addressing priority research directions for precision synthesis with real-time adaptive control.

Stabilized Synthesis of 2D Verbeekite: Monoclinic PdSe2 Crystals with High Mobility and In-Plane Optical and Electrical Anisotropy

PdSe₂ has a layered structure with an unusual, puckered Cairo pentagonal tiling. Its atomic bond configuration features planar 4-fold-coordinated Pd atoms and intralayer Se-Se bonds that enable polymorphic phases with distinct electronic and quantum properties, especially when atomically thin. PdSe₂ is conventionally orthorhombic, and direct synthesis of its metastable polymorphic phases is still a challenge. Here, we report an ambient-pressure chemical vapor deposition approach to synthesize metastable monoclinic PdSe₂. Monoclinic PdSe₂ is shown to be synthesized selectively under Se-deficient conditions that induce Se vacancies. These defects are shown by first-principles density functional theory calculations to reduce the free energy of the metastable monoclinic phase, thereby stabilizing it during synthesis. The structure and composition of the monoclinic PdSe₂ crystals are identified and characterized by scanning transmission electron microscopy imaging, convergent beam electron diffraction, and electron energy loss spectroscopy. Polarized Raman spectroscopy of the monoclinic PdSe₂ flakes reveals their strong in-plane optical anisotropy. Electrical transport measurements show that the monoclinic PdSe₂ exhibits n-type charge carrier conduction with electron mobilities up to ∼298 cm² V^-1 s^-1 and a strong in-plane electron mobility anisotropy of ∼1.9. The defect-mediated growth pathway identified in this work is promising for phase-selective direct synthesis of other 2D transition metal dichalcogenides.

Selective Antisite Defect Formation in WS2 Monolayers via Reactive Growth on Dilute W-Au Alloy Substrates

Defects are ubiquitous in 2D materials and can affect the structure and properties of the materials and also introduce new functionalities. Methods to adjust the structure and density of defects during bottom-up synthesis are required to control the growth of 2D materials with tailored optical and electronic properties. Here, the authors present an Au-assisted chemical vapor deposition approach to selectively form S_W and S2_W antisite defects, whereby one or two sulfur atoms substitute for a tungsten atom in WS₂ monolayers. Guided by first-principles calculations, they describe a new method that can maintain tungsten-poor growth conditions relative to sulfur via the low solubility of W atoms in a gold/W alloy, thereby significantly reducing the formation energy of the antisite defects during the growth of WS₂ . The atomic structure and composition of the antisite defects are unambiguously identified by Z-contrast scanning transmission electron microscopy and electron energy-loss spectroscopy, and their total concentration is statistically determined, with levels up to ≈5.0%. Scanning tunneling microscopy/spectroscopy measurements and first-principles calculations further verified these antisite defects and revealed the localized defect states in the bandgap of WS₂ monolayers. This bottom-up synthesis method to form antisite defects should apply in the synthesis of other 2D materials.

Exploring the Spatial Control of Topotactic Phase Transitions Using Vertically Oriented Epitaxial Interfaces

Engineering oxygen vacancy formation and distribution is a powerful route for controlling the oxygen sublattice evolution that affects diverse functional behavior. The controlling of the oxygen vacancy formation process is particularly important for inducing topotactic phase transitions that occur by transformation of the oxygen sublattice. Here we demonstrate an epitaxial nanocomposite approach for exploring the spatial control of topotactic phase transition from a pristine perovskite phase to an oxygen vacancy-ordered brownmillerite (BM) phase in a model oxide La_0.7Sr_0.3MnO₃ (LSMO). Incorporating a minority phase NiO in LSMO films creates ultrahigh density of vertically aligned epitaxial interfaces that strongly influence the oxygen vacancy formation and distribution in LSMO. Combined structural characterizations reveal strong interactions between NiO and LSMO across the epitaxial interfaces leading to a topotactic phase transition in LSMO accompanied by significant morphology evolution in NiO. Using the NiO nominal ratio as a single control parameter, we obtain intermediate topotactic nanostructures with distinct distribution of the transformed LSMO-BM phase, which enables systematic tuning of magnetic and electrical transport properties. The use of self-assembled heterostructure interfaces by the epitaxial nanocomposite platform enables more versatile design of topotactic phase structures and correlated functionalities that are sensitive to oxygen vacancies.

Understanding Substrate-Guided Assembly in van der Waals Epitaxy by in Situ Laser Crystallization within a Transmission Electron Microscope

Understanding the bottom-up synthesis of atomically thin two-dimensional (2D) crystals and heterostructures is important for the development of new processing strategies to assemble 2D heterostructures with desired functional properties. Here, we utilize in situ laser-heating within a transmission electron microscope (TEM) to understand the stages of crystallization and coalescence of amorphous precursors deposited by pulsed laser deposition (PLD) as they are guided by 2D crystalline substrates into van der Waals (vdW) epitaxial heterostructures. Amorphous clusters of tungsten selenide were deposited by PLD at room temperature onto graphene or MoSe₂ monolayer crystals that were suspended on TEM grids. The precursors were then stepwise evolved into 2D heterostructures with pulsed laser heating treatments within the TEM. The lattice-matching provided by the MoSe₂ substrate is shown to guide the formation of large-domain, heteroepitaxial vdW WSe₂/MoSe₂ bilayers both during the crystallization process via direct templating and after crystallization by assisting the coalescence of nanosized domains through nonclassical particle attachment processes including domain rotation and grain boundary migration. The favorable energetics for domain rotation induced by lattice matching with the substrate were understood from first-principles calculations. These in situ TEM studies of pulsed laser-driven nonequilibrium crystallization phenomena represent a transformational tool for the rapid exploration of synthesis and processing pathways that may occur on extremely different length and time scales and lend insight into the growth of 2D crystals by PLD and laser crystallization.

Strain-Induced Growth of Twisted Bilayers during the Coalescence of Monolayer MoS2 Crystals

Tailoring the grain boundaries (GBs) and twist angles between two-dimensional (2D) crystals are two crucial synthetic challenges to deterministically enable envisioned applications such as moiré excitons, emerging magnetism, or single-photon emission. Here, we reveal how twisted 2D bilayers can be synthesized from the collision and coalescence of two growing monolayer MoS₂ crystals during chemical vapor deposition. The twisted bilayer (TB) moiré angles are found to preserve the misorientation angle (θ) of the colliding crystals. The shapes of the TB regions are rationalized by a kink propagation model that predicts the GB formed by the coalescing crystals. Optical spectroscopy measurements reveal a θ-dependent long-range strain in crystals with stitched grain boundaries and a sharp (θ > 20°) threshold for the appearance of TBs, which relieves this strain. Reactive molecular dynamics simulations explain this strain from the continued growth of the crystals during coalescence due to the insertion of atoms at unsaturated defects along the GB, a process that self-terminates when the defects become saturated. The simulations also reproduce atomic-resolution electron microscopy observations of faceting along the GB, which is shown to arise from the growth-induced long-range strain. These facets align with the axes of the colliding crystals to provide favorable nucleation sites for second-layer growth of a TB with twist angles that preserve the misorientation angle θ. This interplay between strain generation and aligned nucleation provides a synthetic pathway for the growth of TBs with deterministic angles.

Intrinsic Defects in MoS2 Grown by Pulsed Laser Deposition: From Monolayers to Bilayers

Pulsed laser deposition (PLD) can be considered a powerful method for the growth of two-dimensional (2D) transition-metal dichalcogenides (TMDs) into van der Waals heterostructures. However, despite significant progress, the defects in 2D TMDs grown by PLD remain largely unknown and yet to be explored. Here, we combine atomic resolution images and first-principles calculations to reveal the atomic structure of defects, grains, and grain boundaries in mono- and bilayer MoS₂ grown by PLD. We find that sulfur vacancies and Mo_S antisites are the predominant point defects in 2D MoS₂. We predict that the aforementioned point defects are thermodynamically favorable under a Mo-rich/S-poor environment. The MoS₂ monolayers are polycrystalline and feature nanometer size grains connected by a high density of grain boundaries. In particular, the coalescence of nanometer grains results in the formation of 180° mirror twin boundaries consisting of distinct 4- and 8-membered rings. We show that PLD synthesis of bilayer MoS₂ results in various structural symmetries, including AA' and AB, but also turbostratic with characteristic moiré patterns. Moreover, we report on the experimental demonstration of an electron beam-driven transition between the AB and AA' stacking orientations in bilayer MoS₂. These results provide a detailed insight into the atomic structure of monolayer MoS₂ and the role of the grain boundaries on the growth of bilayer MoS₂, which has importance for future applications in optoelectronics.

Two-Dimensional Palladium Diselenide with Strong In-Plane Optical Anisotropy and High Mobility Grown by Chemical Vapor Deposition

Two-dimensional (2D) palladium diselenide (PdSe₂ ) has strong interlayer coupling and a puckered pentagonal structure, leading to remarkable layer-dependent electronic structures and highly anisotropic in-plane optical and electronic properties. However, the lack of high-quality, 2D PdSe₂ crystals grown by bottom-up approaches limits the study of their exotic properties and practical applications. In this work, chemical vapor deposition growth of highly crystalline few-layer (≥2 layers) PdSe₂ crystals on various substrates is reported. The high quality of the PdSe₂ crystals is confirmed by low-frequency Raman spectroscopy, scanning transmission electron microscopy, and electrical characterization. In addition, strong in-plane optical anisotropy is demonstrated via polarized Raman spectroscopy and second-harmonic generation maps of the PdSe₂ flakes. A theoretical model based on kinetic Wulff construction theory and density functional theory calculations is developed and described the observed evolution of "square-like" shaped PdSe₂ crystals into rhombus due to the higher nucleation barriers for stable attachment on the (1,1) and (1,-1) edges, which results in their slower growth rates. Few-layer PdSe₂ field-effect transistors reveal tunable ambipolar charge carrier conduction with an electron mobility up to ≈294 cm² V^-1 s^-1 , which is comparable to that of exfoliated PdSe₂ , indicating the promise of this anisotropic 2D material for electronics.

Low Energy Implantation into Transition-Metal Dichalcogenide Monolayers to Form Janus Structures

Atomically thin two-dimensional (2D) materials face significant energy barriers for synthesis and processing into functional metastable phases such as Janus structures. Here, the controllable implantation of hyperthermal species from pulsed laser deposition (PLD) plasmas is introduced as a top-down method to compositionally engineer 2D monolayers. The kinetic energies of Se clusters impinging on suspended monolayer WS₂ crystals were controlled in the <10 eV/atom range with in situ plasma diagnostics to determine the thresholds for selective top layer replacement of sulfur by selenium for the formation of high quality WSSe Janus monolayers at low (300 °C) temperatures and bottom layer replacement for complete conversion to WSe₂. Atomic-resolution electron microscopy and spectroscopy in tilted geometry confirm the WSSe Janus monolayer. Molecular dynamics simulations reveal that Se clusters implant to form disordered metastable alloy regions, which then recrystallize to form highly ordered structures, demonstrating low-energy implantation by PLD for the synthesis of 2D Janus layers and alloys of variable composition.

Experimental setup combining in situ hard X-ray photoelectron spectroscopy and real-time surface X-ray diffraction for characterizing atomic and electronic structure evolution during complex oxide heterostructure growth

We describe the next-generation system for in situ characterization of a complex oxide thin film and heterostructure growth by pulsed laser deposition (PLD) using synchrotron hard X-rays. The system consists of a PLD chamber mounted on a diffractometer allowing both real-time surface X-ray diffraction (SXRD) and in situ hard X-ray photoelectron spectroscopy (HAXPES). HAXPES is performed in the incident X-ray energy range from 4 to 12 keV using a Scienta EW4000 electron energy analyzer mounted on the PLD chamber fixed parallel with the surface normal. In addition to the standard application mode of HAXPES for disentangling surface from bulk properties, the increased penetration depth of high energy photoelectrons is used for investigation of the electronic structure changes through thin films grown deliberately as variable thickness capping layers. Such heterostructures represent model systems for investigating a variety of critical thickness and dead layer phenomena observed at complex oxide interfaces. In this new mode of operation, in situ HAXPES is used to determine the electronic structure associated with unique structural features identified by real-time SXRD during thin film growth. The system is configured for using both laboratory excitation sources off-line and on-line operation at beamline 33-ID-D at the Advanced Photon Source. We illustrate the performance of the system by preliminary scattering and spectroscopic data on oxygen vacancy ordering induced perovskite-to-brownmillerite reversible phase transformation in La_2/3Sr_1/3MnO₃ films capped with oxygen deficient SrTiO_3-δ (100) layers of varying thickness.

Defect-Mediated Phase Transformation in Anisotropic Two-Dimensional PdSe2 Crystals for Seamless Electrical Contacts

The failure to achieve stable Ohmic contacts in two-dimensional material devices currently limits their promised performance and integration. Here we demonstrate that a phase transformation in a region of a layered semiconductor, PdSe₂, can form a contiguous metallic Pd₁₇Se₁₅ phase, leading to the formation of seamless Ohmic contacts for field-effect transistors. This phase transition is driven by defects created by exposure to an argon plasma. Cross-sectional scanning transmission electron microscopy is combined with theoretical calculations to elucidate how plasma-induced Se vacancies mediate the phase transformation. The resulting Pd₁₇Se₁₅ phase is stable and shares the same native chemical bonds with the original PdSe₂ phase, thereby forming an atomically sharp Pd₁₇Se₁₅/PdSe₂ interface. These Pd₁₇Se₁₅ contacts exhibit a low contact resistance of ∼0.75 kΩ μm and Schottky barrier height of ∼3.3 meV, enabling nearly a 20-fold increase of carrier mobility in PdSe₂ transistors compared to that of traditional Ti/Au contacts. This finding opens new possibilities in the development of better electrical contacts for practical applications of 2D materials.

Low-Temperature Charging Dynamics of the Ionic Liquid and Its Gating Effect on FeSe0.5Te0.5 Superconducting Films

Ionic liquids (ILs) have been investigated extensively because of their unique ability to form the electric double layer (EDL), which induces high electrical field. For certain materials, low-temperature IL charging is needed to limit the electrochemical etching. Here, we report our investigation of the low-temperature charging dynamics in two widely used ILs-DEME-TF₂N and C₄mim-TF₂N. Results show that the formation of the EDL at ∼220 K requires several hours relative to milliseconds at room temperature, and an equivalent voltage V_e is introduced as a measure of the EDL formation during the biasing process. The experimental observation is supported by molecular dynamics simulation, which shows that the dynamics are logically a function of gate voltage, time, and temperature. To demonstrate the importance of understanding the charging dynamics, a 140 nm thick FeSe_0.5Te_0.5 film was biased using the DEME IL, showing a tunable T_c between 18 and 35 K. Notably, this is the first observation of the tunability of the T_c in thick film FeSe_0.5Te_0.5 superconductors.

Strain tolerance of two-dimensional crystal growth on curved surfaces

Two-dimensional (2D) crystal growth over substrate features is fundamentally guided by the Gauss-Bonnet theorem, which mandates that rigid, planar crystals cannot conform to surfaces with nonzero Gaussian curvature. Here, we reveal how topographic curvature of lithographically designed substrate features govern the strain and growth dynamics of triangular WS₂ monolayer single crystals. Single crystals grow conformally without strain over deep trenches and other features with zero Gaussian curvature; however, features with nonzero Gaussian curvature can easily impart sufficient strain to initiate grain boundaries and fractured growth in different directions. Within a strain-tolerant regime, however, triangular single crystals can accommodate considerable (<1.1%) localized strain exerted by surface features that shift the bandgap up to 150 meV. Within this regime, the crystal growth accelerates in specific directions, which we describe using a growth model. These results present a previously unexplored strategy to strain-engineer the growth directions and optoelectronic properties of 2D crystals.

2D/2D heterojunction of Ti3C2/g-C3N4 nanosheets for enhanced photocatalytic hydrogen evolution

Photocatalytic hydrogen evolution from water has received enormous attention due to its ability to address a number of global environmental and energy-related issues. Here, we synthesize 2D/2D Ti3C2/g-C3N4 composites by electrostatic self-assembly technique and demonstrate their use as photocatalysts for hydrogen evolution under visible light irradiation. The optimized Ti3C2/g-C3N4 composite exhibited a 10 times higher photocatalytic hydrogen evolution performance (72.3 μmol h-1 gcat-1) than that of pristine g-C3N4 (7.1 μmol h-1 gcat-1). Such enhanced photocatalytic performance was due to the formation of 2D/2D heterojunctions in the Ti3C2/g-C3N4 composites. The intimate contact between the monolayer Ti3C2 and g-C3N4 nanosheets promotes the separation of photogenerated charge carriers at the Ti3C2/g-C3N4 interface. Furthermore, the ultrahigh conductivity of Ti3C2 and the Schottky junction formed between g-C3N4/MXene interfaces facilitate the photoinduced electron transfer and suppress the recombination with photogenerated holes. This work demonstrates that the 2D/2D Ti3C2/g-C3N4 composites are promising photocatalysts thanks to the ultrathin MXenes as efficient co-catalysts for photocatalytic hydrogen production.

Isotope-Engineering the Thermal Conductivity of Two-Dimensional MoS2

Isotopes represent a degree of freedom that might be exploited to tune the physical properties of materials while preserving their chemical behaviors. Here, we demonstrate that the thermal properties of two-dimensional (2D) transition-metal dichalcogenides can be tailored through isotope engineering. Monolayer crystals of MoS₂ were synthesized with isotopically pure ¹⁰⁰Mo and ⁹²Mo by chemical vapor deposition employing isotopically enriched molybdenum oxide precursors. The in-plane thermal conductivity of the ¹⁰⁰MoS₂ monolayers, measured using a non-destructive, optothermal Raman technique, is found to be enhanced by ∼50% compared with the MoS₂ synthesized using mixed Mo isotopes from naturally occurring molybdenum oxide. The boost of thermal conductivity in isotopically pure MoS₂ monolayers is attributed to the combined effects of reduced isotopic disorder and a reduction in defect-related scattering, consistent with observed stronger photoluminescence and longer exciton lifetime. These results shed light on the fundamentals of 2D nanoscale thermal transport important for the optimization of 2D electronic devices.

In situ edge engineering in two-dimensional transition metal dichalcogenides

Exerting synthetic control over the edge structure and chemistry of two-dimensional (2D) materials is of critical importance to direct the magnetic, optical, electrical, and catalytic properties for specific applications. Here, we directly image the edge evolution of pores in Mo1-xW x Se2 monolayers via atomic-resolution in situ scanning transmission electron microscopy (STEM) and demonstrate that these edges can be structurally transformed to theoretically predicted metastable atomic configurations by thermal and chemical driving forces. Density functional theory calculations and ab initio molecular dynamics simulations explain the observed thermally induced structural evolution and exceptional stability of the four most commonly observed edges based on changing chemical potential during thermal annealing. The coupling of modeling and in situ STEM imaging in changing chemical environments demonstrated here provides a pathway for the predictive and controlled atomic scale manipulation of matter for the directed synthesis of edge configurations in Mo1-x W x Se2 to achieve desired functionality.

Real-Time Observation of Order-Disorder Transformation of Organic Cations Induced Phase Transition and Anomalous Photoluminescence in Hybrid Perovskites

A fundamental understanding of the interplay between the microscopic structure and macroscopic optoelectronic properties of organic-inorganic hybrid perovskite materials is essential to design new materials and improve device performance. However, how exactly the organic cations affect the structural phase transition and optoelectronic properties of the materials is not well understood. Here, real-time, in situ temperature-dependent neutron/X-ray diffraction and photoluminescence (PL) measurements reveal a transformation of the organic cation CH₃ NH₃⁺ from order to disorder with increasing temperature in CH₃ NH₃ PbBr₃ perovskites. The molecular-level order-to-disorder transformation of CH₃ NH₃⁺ not only leads to an anomalous increase in PL intensity, but also results in a multidomain to single-domain structural transition. This discovery establishes the important role that organic cation ordering has in dictating structural order and anomalous optoelectronic phenomenon in hybrid perovskites.

Influence of Nonstoichiometry on Proton Conductivity in Thin-Film Yttrium-Doped Barium Zirconate

Proton-conducting perovskites have been widely studied because of their potential application as solid electrolytes in intermediate temperature solid oxide fuel cells. Structural and chemical heterogeneities can develop during synthesis, device fabrication, or service, which can profoundly affect proton transport. Here, we use time-resolved Kelvin probe force microscopy, scanning transmission electron microscopy, atom probe tomography, and density functional theory calculations to intentionally introduce Ba-deficient planar and spherical defects and link the resultant atomic structure with proton transport behavior in both stoichiometric and nonstoichiometric epitaxial, yttrium-doped barium zirconate thin films. The defects were intentionally induced through high-temperature annealing treatment, while maintaining the epitaxial single crystalline structure of the films, with an overall relaxation in the atomic structure. The annealed samples showed smaller magnitudes of local lattice distortions because of the formation of proton polarons, thereby leading to decreased proton-trapping effect. This resulted in a decrease in the activation energy for proton transport, leading to faster proton transport.

Nonequilibrium Synthesis of TiO2 Nanoparticle "Building Blocks" for Crystal Growth by Sequential Attachment in Pulsed Laser Deposition

Nonequilibrium growth pathways for crystalline nanostructures with metastable phases are demonstrated through the gas-phase formation, attachment, and crystallization of ultrasmall amorphous nanoparticles as building blocks in pulsed laser deposition (PLD). Temporally and spatially resolved gated-intensified charge couple device (ICCD) imaging and ion probe measurements are employed as in situ diagnostics to understand and control the plume expansion conditions for the synthesis of nearly pure fluxes of ultrasmall (∼3 nm) amorphous TiO₂ nanoparticles in background gases and their selective delivery to substrates. These amorphous nanoparticles assemble into loose, mesoporous assemblies on substrates at room temperature but dynamically crystallize by sequential particle attachment at higher substrate temperatures to grow nanostructures with different phases and morphologies. Molecular dynamics calculations are used to simulate and understand the crystallization dynamics. This work demonstrates that nonequilibrium crystallization by particle attachment of metastable ultrasmall nanoscale "building blocks" provides a versatile approach for exploring and controlling the growth of nanoarchitectures with desirable crystalline phases and morphologies.

Black Anatase Formation by Annealing of Amorphous Nanoparticles and the Role of the Ti2O3 Shell in Self-Organized Crystallization by Particle Attachment

We use amorphous titania nanoparticle networks produced by pulsed laser vaporization at room temperature as a model system for understanding the mechanism of formation of black titania. Here, we characterize the transformation of amorphous nanoparticles by annealing in pure Ar at 400 °C, the lowest temperature at which black titania was observed. Atomic resolution electron microscopy methods and electron energy loss spectroscopy show that the onset of crystallization occurs by nucleation of an anatase core that is surrounded by an amorphous Ti₂O₃ shell. The formation of the metastable anatase core before the thermodynamically stable rutile phase occurs according to the Ostwald phase rule. In the second stage the particle size increases by coalescence of already crystallized particles by a self-organized mechanism of crystallization by particle attachment. We show that the Ti₂O₃ shell plays a critical role in both black titania transformation and functionality. At 400 °C, Ti₂O₃ hinders the agglomeration of neighboring particles to maintain a high surface-to-volume ratio that is beneficial for enhanced photocatalytic activity. In agreement with previous results, the thin Ti₂O₃ surface layer acts as a narrow bandgap semiconductor in concert with surface defects to enhance the photocatalytic activity. Our results demonstrate that crystallization by particle attachment can be a highly effective mechanism for optimizing photocatalytic efficiency by controlling the phase, composition, and particle size distribution in a wide range of self-doped defective TiO₂ architectures simply by varying the annealing conditions of amorphous nanoparticles.

Layer-by-layer epitaxial thin films of the pyrochlore Tb2Ti2O7

Layer-by-layer epitaxial growth of the pyrochlore magnet Tb₂Ti₂O₇ on the isostructural substrate Y₂Ti₂O₇ results in high-quality single crystal films of up to 60 nm thickness. Substrate-induced strain is shown to act as a strong and controlled perturbation to the exotic magnetism of Tb₂Ti₂O₇, opening up the general prospect of strain-engineering the diverse magnetic and electrical properties of pyrochlore oxides.

Observation of Nanoscale Morphological and Structural Degradation in Perovskite Solar Cells by in Situ TEM

High-resolution in situ transmission electron microscopy (TEM) and electron energy loss spectroscopy were applied to systematically investigate morphological and structural degradation behaviors in perovskite films during different environmental exposure treatments. In situ TEM experiment indicates that vacuum itself is not likely to cause degradation in perovskites. In addition, these materials were found to degrade significantly when they were heated to ∼50-60 °C (i.e., a solar cell's field operating temperature) under illumination. This observation thus conveys a critically important message that the instability of perovskite solar cells at such a low temperature may limit their real field commercial applications. It was further unveiled that oxygen most likely attacks the CH₃NH₃⁺ organic moiety rather than the PbI₆ component of perovskites during ambient air exposure at room temperature. This finding grants a deeper understanding of the perovskite degradation mechanism and suggests a way to prevent degradation of perovskites by tailoring the organic moiety component.

Dynamic Scaling and Island Growth Kinetics in Pulsed Laser Deposition of SrTiO_{3}

We use real-time diffuse surface x-ray diffraction to probe the evolution of island size distributions and its effects on surface smoothing in pulsed laser deposition (PLD) of SrTiO_{3}. We show that the island size evolution obeys dynamic scaling and two distinct regimes of island growth kinetics. Our data show that PLD film growth can persist without roughening despite thermally driven Ostwald ripening, the main mechanism for surface smoothing, being shut down. The absence of roughening is concomitant with decreasing island density, contradicting the prevailing view that increasing island density is the key to surface smoothing in PLD. We also report a previously unobserved crossover from diffusion-limited to attachment-limited island growth that reveals the influence of nonequilibrium atomic level surface transport processes on the growth modes in PLD. We show by direct measurements that attachment-limited island growth is the dominant process in PLD that creates step flowlike behavior or quasistep flow as PLD "self-organizes" local step flow on a length scale consistent with the substrate temperature and PLD parameters.

Isoelectronic Tungsten Doping in Monolayer MoSe2 for Carrier Type Modulation

Carrier-type modulation is demonstrated in 2D transition metal dichalcogenides as n-type monolayer MoSe₂ is converted to nondegenerate p-type monolayer Mo_1-x W_x Se₂ through isoelectronic doping. Although the alloys are mesoscopically uniform, the p-type conduction in monolayer Mo_1-x W_x Se₂ appears to originate from the upshift of the valenceband maximum toward the Fermi level at highly localized "W-rich" regions in the lattice.

Tailoring Vacancies Far Beyond Intrinsic Levels Changes the Carrier Type and Optical Response in Monolayer MoSe2-x Crystals

Defect engineering has been a critical step in controlling the transport characteristics of electronic devices, and the ability to create, tune, and annihilate defects is essential to enable the range of next-generation devices. Whereas defect formation has been well-demonstrated in three-dimensional semiconductors, similar exploration of the heterogeneity in atomically thin two-dimensional semiconductors and the link between their atomic structures, defects, and properties has not yet been extensively studied. Here, we demonstrate the growth of MoSe2-x single crystals with selenium (Se) vacancies far beyond intrinsic levels, up to ∼20%, that exhibit a remarkable transition in electrical transport properties from n- to p-type character with increasing Se vacancy concentration. A new defect-activated phonon band at ∼250 cm(-1) appears, and the A1g Raman characteristic mode at 240 cm(-1) softens toward ∼230 cm(-1) which serves as a fingerprint of vacancy concentration in the crystals. We show that post-selenization using pulsed laser evaporated Se atoms can repair Se-vacant sites to nearly recover the properties of the pristine crystals. First-principles calculations reveal the underlying mechanisms for the corresponding vacancy-induced electrical and optical transitions.

Interlayer Coupling in Twisted WSe2/WS2 Bilayer Heterostructures Revealed by Optical Spectroscopy

van der Waals (vdW) heterostructures are promising building blocks for future ultrathin electronics. Fabricating vdW heterostructures by stamping monolayers at arbitrary angles provides an additional range of flexibility to tailor the resulting properties than could be expected by direct growth. Here, we report fabrication and comprehensive characterizations of WSe2/WS2 bilayer heterojunctions with various twist angles that were synthesized by artificially stacking monolayers of WS2 and WSe2 grown by chemical vapor deposition. After annealing the WSe2/WS2 bilayers, Raman spectroscopy reveals interlayer coupling with the appearance of a mode at 309.4 cm(-1) that is sensitive to the number of WSe2 layers. This interlayer coupling is associated with substantial quenching of the intralayer photoluminescence. In addition, microabsorption spectroscopy of WSe2/WS2 bilayers revealed spectral broadening and shifts as well as a net ∼10% enhancement in integrated absorption strength across the visible spectrum with respect to the sum of the individual monolayer spectra. The observed broadening of the WSe2 A exciton absorption band in the bilayers suggests fast charge separation between the layers, which was supported by direct femtosecond pump-probe spectroscopy. Density functional calculations of the band structures of the bilayers at different twist angles and interlayer distances found robust type II heterojunctions at all twist angles, and predicted variations in band gap for particular atomistic arrangements. Although interlayer excitons were indicated using femtosecond pump-probe spectroscopy, photoluminescence and absorption spectroscopies did not show any evidence of them, suggesting that the interlayer exciton transition is very weak. However, the interlayer coupling for the WSe2/WS2 bilayer heterojunctions indicated by substantial PL quenching, enhanced absorption, and rapid charge transfer was found to be insensitive to the relative twist angle, indicating that stamping provides a robust approach to realize reliable optoelectronics.

Thickness-dependent charge transport in few-layer MoS₂ field-effect transistors

Molybdenum disulfide (MoS2) is currently under intensive study because of its exceptional optical and electrical properties in few-layer form. However, how charge transport mechanisms vary with the number of layers in MoS2 flakes remains unclear. Here, exfoliated flakes of MoS2 with various thicknesses were successfully fabricated into field-effect transistors (FETs) to measure the thickness and temperature dependences of electrical mobility. For these MoS2 FETs, measurements at both 295 K and 77 K revealed the maximum mobility for layer thicknesses between 5 layers (∼3.6 nm) and 10 layers (∼7 nm), with ∼70 cm(2) V(-1) s(-1) measured for 5 layer devices at 295 K. Temperature-dependent mobility measurements revealed that the mobility rises with increasing temperature to a maximum. This maximum occurs at increasing temperature with increasing layer thickness, possibly due to strong Coulomb scattering from charge impurities or weakened electron-phonon interactions for thicker devices. Temperature-dependent conductivity measurements for different gate voltages revealed a metal-to-insulator transition for devices thinner than 10 layers, which may enable new memory and switching applications. This study advances the understanding of fundamental charge transport mechanisms in few-layer MoS2, and indicates the promise of few-layer transition metal dichalcogenides as candidates for potential optoelectronic applications.

Two-dimensional GaSe/MoSe2 misfit bilayer heterojunctions by van der Waals epitaxy

Two-dimensional (2D) heterostructures hold the promise for future atomically thin electronics and optoelectronics because of their diverse functionalities. Although heterostructures consisting of different 2D materials with well-matched lattices and novel physical properties have been successfully fabricated via van der Waals (vdW) epitaxy, constructing heterostructures from layered semiconductors with large lattice misfits remains challenging. We report the growth of 2D GaSe/MoSe2 heterostructures with a large lattice misfit using two-step chemical vapor deposition (CVD). Both vertically stacked and lateral heterostructures are demonstrated. The vertically stacked GaSe/MoSe2 heterostructures exhibit vdW epitaxy with well-aligned lattice orientation between the two layers, forming a periodic superlattice. However, the lateral heterostructures exhibit no lateral epitaxial alignment at the interface between GaSe and MoSe2 crystalline domains. Instead of a direct lateral connection at the boundary region where the same lattice orientation is observed between GaSe and MoSe2 monolayer domains in lateral GaSe/MoSe2 heterostructures, GaSe monolayers are found to overgrow MoSe2 during CVD, forming a stripe of vertically stacked vdW heterostructures at the crystal interface. Such vertically stacked vdW GaSe/MoSe2 heterostructures are shown to form p-n junctions with effective transport and separation of photogenerated charge carriers between layers, resulting in a gate-tunable photovoltaic response. These GaSe/MoSe2 vdW heterostructures should have applications as gate-tunable field-effect transistors, photodetectors, and solar cells.

Full Electroresistance Modulation in a Mixed-Phase Metallic Alloy

We report a giant, ∼22%, electroresistance modulation for a metallic alloy above room temperature. It is achieved by a small electric field of 2  kV/cm via piezoelectric strain-mediated magnetoelectric coupling and the resulting magnetic phase transition in epitaxial FeRh/BaTiO_{3} heterostructures. This work presents detailed experimental evidence for an isothermal magnetic phase transition driven by tetragonality modulation in FeRh thin films, which is in contrast to the large volume expansion in the conventional temperature-driven magnetic phase transition in FeRh. Moreover, all the experimental results in this work illustrate FeRh as a mixed-phase model system well similar to phase-separated colossal magnetoresistance systems with phase instability therein.

Carbon Nanotubes Grown on Metal Microelectrodes for the Detection of Dopamine

Microelectrodes modified with carbon nanotubes (CNTs) are useful for the detection of neurotransmitters because the CNTs enhance sensitivity and have electrocatalytic effects. CNTs can be grown on carbon fiber microelectrodes (CFMEs) but the intrinsic electrochemical activity of carbon fibers makes evaluating the effect of CNT enhancement difficult. Metal wires are highly conductive and many metals have no intrinsic electrochemical activity for dopamine, so we investigated CNTs grown on metal wires as microelectrodes for neurotransmitter detection. In this work, we successfully grew CNTs on niobium substrates for the first time. Instead of planar metal surfaces, metal wires with a diameter of only 25 μm were used as CNT substrates; these have potential in tissue applications due to their minimal tissue damage and high spatial resolution. Scanning electron microscopy shows that aligned CNTs are grown on metal wires after chemical vapor deposition. By use of fast-scan cyclic voltammetry, CNT-coated niobium (CNT-Nb) microelectrodes exhibit higher sensitivity and lower ΔEp value compared to CNTs grown on carbon fibers or other metal wires. The limit of detection for dopamine at CNT-Nb microelectrodes is 11 ± 1 nM, which is approximately 2-fold lower than that of bare CFMEs. Adsorption processes were modeled with a Langmuir isotherm, and detection of other neurochemicals was also characterized, including ascorbic acid, 3,4-dihydroxyphenylacetic acid, serotonin, adenosine, and histamine. CNT-Nb microelectrodes were used to monitor stimulated dopamine release in anesthetized rats with high sensitivity. This study demonstrates that CNT-grown metal microelectrodes, especially CNTs grown on Nb microelectrodes, are useful for monitoring neurotransmitters.

Low temperature synthesis of hierarchical TiO2 nanostructures for high performance perovskite solar cells by pulsed laser deposition

High aspect-ratio TiO₂ nanostructures directly assembled with pulsed laser deposition could improve interfacial contact for superior perovskite photovoltaic cells.

Structure and Formation Mechanism of Black TiO2 Nanoparticles

The remarkable properties of black TiO2 are due to its disordered surface shell surrounding a crystalline core. However, the chemical composition and the atomic and electronic structure of the disordered shell and its relationship to the core remain poorly understood. Using advanced transmission electron microscopy methods, we show that the outermost layer of black TiO2 nanoparticles consists of a disordered Ti2O3 shell. The measurements show a transition region that connects the disordered Ti2O3 shell to the perfect rutile core consisting first of four to five monolayers of defective rutile, containing clearly visible Ti interstitial atoms, followed by an ordered reconstruction layer of Ti interstitial atoms. Our data suggest that this reconstructed layer presents a template on which the disordered Ti2O3 layers form by interstitial diffusion of Ti ions. In contrast to recent reports that attribute TiO2 band-gap narrowing to the synergistic action of oxygen vacancies and surface disorder of nonspecific origin, our results point to Ti2O3, which is a narrow-band-gap semiconductor. As a stoichiometric compound of the lower oxidation state Ti(3+) it is expected to be a more robust atomic structure than oxygen-deficient TiO2 for preserving and stabilizing Ti(3+) surface species that are the key to the enhanced photocatalytic activity of black TiO2.

Van der Waals Epitaxial Growth of Two-Dimensional Single-Crystalline GaSe Domains on Graphene

Two-dimensional (2D) van der Waals (vdW) heterostructures are a family of artificially structured materials that promise tunable optoelectronic properties for devices with enhanced functionalities. Compared to transferring, direct epitaxy of vdW heterostructures is ideal for clean interlayer interfaces and scalable device fabrication. Here we report the synthesis and preferred orientations of 2D GaSe atomic layers on graphene (Gr) by vdW epitaxy. GaSe crystals are found to nucleate predominantly on random wrinkles or grain boundaries of graphene, share a preferred lattice orientation with underlying graphene, and grow into large (tens of micrometers) irregularly shaped, single-crystalline domains. The domains are found to propagate with triangular edges that merge into the large single crystals during growth. Electron diffraction reveals that approximately 50% of the GaSe domains are oriented with a 10.5 ± 0.3° interlayer rotation with respect to the underlying graphene. Theoretical investigations of interlayer energetics reveal that a 10.9° interlayer rotation is the most energetically preferred vdW heterostructure. In addition, strong charge transfer in these GaSe/Gr vdW heterostructures is predicted, which agrees with the observed enhancement in the Raman E(2)1g band of monolayer GaSe and highly quenched photoluminescence compared to GaSe/SiO2. Despite the very large lattice mismatch of GaSe/Gr through vdW epitaxy, the predominant orientation control and convergent formation of large single-crystal flakes demonstrated here is promising for the scalable synthesis of large-area vdW heterostructures for the development of new optical and optoelectronic devices.

Digital transfer growth of patterned 2D metal chalcogenides by confined nanoparticle evaporation

Developing methods for the facile synthesis of two-dimensional (2D) metal chalcogenides and other layered materials is crucial for emerging applications in functional devices. Controlling the stoichiometry, number of the layers, crystallite size, growth location, and areal uniformity is challenging in conventional vapor-phase synthesis. Here, we demonstrate a method to control these parameters in the growth of metal chalcogenide (GaSe) and dichalcogenide (MoSe2) 2D crystals by precisely defining the mass and location of the source materials in a confined transfer growth system. A uniform and precise amount of stoichiometric nanoparticles are first synthesized and deposited onto a substrate by pulsed laser deposition (PLD) at room temperature. This source substrate is then covered with a receiver substrate to form a confined vapor transport growth (VTG) system. By simply heating the source substrate in an inert background gas, a natural temperature gradient is formed that evaporates the confined nanoparticles to grow large, crystalline 2D nanosheets on the cooler receiver substrate, the temperature of which is controlled by the background gas pressure. Large monolayer crystalline domains (∼100 μm lateral sizes) of GaSe and MoSe2 are demonstrated, as well as continuous monolayer films through the deposition of additional precursor materials. This PLD-VTG synthesis and processing method offers a unique approach for the controlled growth of large-area metal chalcogenides with a controlled number of layers in patterned growth locations for optoelectronics and energy related applications.

Cooperative island growth of large-area single-crystal graphene on copper using chemical vapor deposition

In this work we explore the kinetics of single-crystal graphene growth as a function of nucleation density. In addition to the standard methods for suppressing nucleation of graphene by pretreatment of Cu foils using oxidation, annealing, and reduction of the Cu foils prior to growth, we introduce a new method that further reduces the graphene nucleation density by interacting directly with the growth process at the onset of nucleation. The successive application of these two methods results in roughly 3 orders of magnitude reduction in graphene nucleation density. We use a kinetic model to show that at vanishingly low nucleation densities carbon incorporation occurs by a cooperative island growth mechanism that favors the formation of substrate-size single-crystal graphene. The model reveals that the cooperative growth of millimeter-size single-crystal graphene grains occurs by roughly 3 orders of magnitude increase in the reactive sticking probability of methane compared to that in random island nucleation.

Real-time optical diagnostics of graphene growth induced by pulsed chemical vapor deposition

The kinetics and mechanisms of graphene growth on Ni films at 720-880 °C have been measured using fast pulses of acetylene and real-time optical diagnostics. In situ UV-Raman spectroscopy was used to unambiguously detect isothermal graphene growth at high temperatures, measure the growth kinetics with ∼1 s temporal resolution, and estimate the fractional precipitation upon cooldown. Optical reflectivity and videography provided much faster temporal resolution. Both the growth kinetics and the fractional isothermal precipitation were found to be governed by the C2H2 partial pressure in the CVD pulse for a given film thickness and temperature, with up to ∼94% of graphene growth occurring isothermally within 1 second at 800 °C at high partial pressures. At lower partial pressures, isothermal graphene growth is shown to continue 10 seconds after the gas pulse. These flux-dependent growth kinetics are described in the context of a dissolution/precipitation model, where carbon rapidly dissolves into the Ni film and later precipitates driven by gradients in the chemical potential. The combination of pulsed-CVD and real-time optical diagnostics opens new opportunities to understand and control the fast, sub-second growth of graphene on various substrates at high temperatures.